1. Field of the Invention
The subject matter disclosed herein relates generally to communication over a telephone loop; and, more particularly, to a differential transformer-free hybrid circuit that eliminates the use of a transformer to interface the hybrid circuit to a telephone loop in a communications system.
2. Description of the Prior Art
A hybrid circuit is a circuit that converts between a bidirectional transmission on a single wire-pair and two unidirectional transmissions on two wire-pairs. A hybrid circuit used for transmission of audio receives audio signals from a telephone loop, outputs audio signals to a receiver wire-pair, and transmits audio signals onto the telephone loop that the circuit receives from a transmitter wire-pair. Because hybrid circuits interface with a telephone loop having an often unknown impedance, the hybrid circuits are designed for a wide range of line impedance levels. When one hybrid circuit transmits a signal over the telephone loop, the signal may face an impedance discontinuity due to a presence of an imperfect hybrid at an opposite end of the telephone loop, unknown line taps, or an unknown number of phones attached to the loop, for example. This impedance discontinuity creates signal reflections, which can leak into the incoming audio signal path and manifest themselves as audible echoes in the audio signal transmissions over the telephone loops because the hybrid circuit receives the signal reflection and unknowingly treats it as a part of the received signal. Echoes in audio communication are undesirable, even more so if the echoes are perceived by a user, which may occur if a round-trip delay of the echo exceeds a few tens of milliseconds and is unattenuated or only slightly attenuated.
Current technology hybrid circuits also require transformers to interface with the telephone loop. Transformer-based circuit constructs can be disadvantaged by these often bulky and expensive transformers. Use of transformers may result in limited frequency response of the overall telephone loop bi-directional transmissions, audible magnetic coupling noise from the transformers, and/or relatively large power consumption for short loops.
If an impedance of a telephone loop is controlled to within a certain range, or measured and/or modeled more accurately, hybrid circuits may be designed to more closely match the corresponding telephone loop impedance; thus, avoiding or minimizing echoes in the system.
Similarly, if a hybrid circuit can be created that eliminates the use of a transformer to interface with the telephone loop, the hybrid circuit may be created at a lower cost, lower weight, and/or may reduce, if not eliminate, problems associated with the use of a transformer in such a hybrid circuit.
FIG. 1 is a schematic diagram of a conventional telephone audio communication system 100 including a first hybrid circuit 104 and a second hybrid circuit 108. First hybrid circuit 104 receives an audio signal from a first transmitter (not shown) through a first amplifier 112 and transmits the audio signal over a telephone loop 116 to second hybrid circuit 108. Second hybrid circuit 108 receives the audio signal from telephone loop 116 and outputs the audio signal to a first receiver (not shown) using a second amplifier 120. Second hybrid circuit 108 also receives an audio signal from a second transmitter (not shown) through a third amplifier 124 and transmits the audio signal over telephone loop 116 to first hybrid circuit 104. First hybrid circuit 104 receives the audio signal from telephone loop 116 and outputs the audio signal to a second receiver using a fourth amplifier 128.
FIG. 2 is a partial schematic diagram of a hybrid circuit 200, such as first hybrid circuit 204 shown in FIG. 1. A first audio signal (not shown) is inputted to hybrid circuit 200 from the transmitter through a first operational amplifier 204, which outputs the amplified first audio signal to a first node 208. A voltage drop of the first audio signal is accomplished by a first balanced network element (Z1) 212 electrically coupling first node 208 to second node 216, and a second balanced network element (Z2) 224 electrically coupling second node 216 to ground. Similarly, a voltage drop of the first audio signal is accomplished across a third balanced network element (ZB) 228 and a line transformer 232 with a transformer line impedance (ZL) 236. The impedance ratio of first balanced network element 212 to second balanced network element 224, is approximately proportional to the impedance ratio of third balanced network element 228 to transformer line impedance 236 as to enable an approximately proportional voltage drop across each element. Hybrid circuit 200 provides for the amplified first audio signal to be output through line transformer 232. Hybrid circuit 200 also provides for a second audio signal (not shown) to be received from the telephone loop through line transformer 232 and output to the receiver through second operational amplifier 220. Analysis of hybrid circuit 200 reveals that it appears to null the echo only for the several typical cases where the telephone loop configuration is selected and simplified. To better provide for echo cancellation, a full set of loop configurations should be scanned and a worst case should be identified to enable a more robust echo cancellation circuit.